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Conservatives often point out that laws, no matter how benign they may appear, have unintended consequences. They can reverberate in ways that not many people foresaw and nobody wanted: Raising the minimum wage can increase unemployment; prohibition can create black markets.

The efforts in many cities to discourage the use of plastic bags demonstrate that such unintended consequences can be, among other things, kind of gross.

San Francisco has been discouraging plastic bags since 2007, saying that it takes too much oil to make them and that used bags pollute waterways and kill marine animals. In 2012, it strengthened its law.

Several West Coast cities, including Seattle and Los Angeles, have also adopted bans for environmental reasons. Washington, D.C., imposes a 5 cent plastic-bag tax. (Advocates prefer to call it a “fee” because taxes are unpopular.) Environmental groups and celebrity activists, including Eva Longoria and Julia Louis- Dreyfus, support these laws.

The plastic-bag industry, predictably, wants to throw them away. It cites a 2007 report by San Francisco’s Environment Department that said plastic bags from retail establishments, the target of the ban, accounted for only 0.6 percent of litter.

And the industry has highlighted news reports linking reusable shopping bags to the spread of disease. Like this one, from the Los Angeles Times last May: “A reusable grocery bag left in a hotel bathroom caused an outbreak of norovirus-induced diarrhea and nausea that struck nine of 13 members of a girls’ soccer team in October, Oregon researchers reported Wednesday.”

The norovirus may not have political clout, but evidently it, too, is rooting against plastic bags.

An over-the-top scare tactic? Research suggest it’s more than just an industry talking point.

In a 2011 study, four researchers examined reusable bags in California and Arizona and found that 51 percent of them contained coliform bacteria.

The problem appears to be the habits of the reusers: 75 percent said they keep meat and vegetables in the same bag. When bags were stored in hot car trunks for two hours, the bacteria grew tenfold.

That study also found, happily, that washing the bags eliminated 99.9 percent of the bacteria. It undercut even that good news, though, by finding that 97 percent of people reported that they never wash their bags.

Jonathan Klick and Joshua Wright (law professors at the University of Pennsylvania and George Mason University, respectively) have done a more recent study on the public-health impact of plastic-bag bans. They find that emergency-room admissions related to E. coli infections increased in San Francisco after the ban. (Nearby counties did not show this increase.) And this effect showed up as soon as the ban was implemented. The San Francisco ban was also associated with increases in salmonella and other bacterial infections. Similar effects were found in other California towns that adopted such laws.

Klick and Wright estimate that the San Francisco ban results in a 46 percent increase in deaths from foodborne illnesses, or 5.5 more of them each year.

The authors argue, not completely convincingly, against the idea that regular washing and drying of reusable bags would solve the problem. They point out that the use of hot water and detergent imposes environmental costs, too. And reusable bags require more energy to make than plastic ones.

The stronger argument, it seems to me, is that 97 percent figure: Whatever the merits of regularly cleaning the bags, it doesn’t appear likely to happen.

The best course for government, then, is probably to encourage people to recycle their plastic bags — or, maybe, just let people make their own decisions.

Plastic-bag bans are another on a distressingly long list of political issues where I cannot see eye to eye with Eva Longoria.